Excellent advice above. At 4 months, regression is not really that uncommon. Definitely worth trying baby formula, if for no other reason, to get some nutrition into her. Even adults will benefit from it if they're sick and need extra nutritional support. Otherwise, have you tried anything like brown rice or oatmeal? Usually well liked, and lots of calories, which she likely needs right now.
Now, assuming she gets back on track, you should really look into the specifics of ekkie diet. It's important to get it right, and it's not negotiable. Grains are really important, veggies, and a small amount of fruit. Pellets are commonly fortified, and ekkies do not generally do well with the added vitamins. Seeds, not considered good for them, though a very small amount shouldn't be a problem. My guy gets maybe a teaspoon or so full per day. Some foods are debatable. While it's frowned upon by many, some ekkies do okay with some corn in their diet. Mine does not, and will start toe tapping within hours of eating a pretty small amount. Same with blueberries and purple grapes, though my first ekkie was fine with them, and loved them. Okay, he just loved to eat.
There's much more to the diet than what I've mentioned, so like I said, please research. Here's a great place to start.
http://eclectusparrots.net/fruitandveg.html
Best of luck to you, and prayers for your baby girl. I do hope you can get her to an avian vet ASAP as they can best diagnose what's going on.
